Fair value measurements - Schedule of Assets and Liabilities Not Measured at Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2024	Dec. 31, 2023
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Securities purchased under agreements to resell	$ 0	$ 5,842
Corporate-owned life insurance	98,068	88,989
Level 1
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Securities purchased under agreements to resell		0
Level 2
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Securities purchased under agreements to resell		5,842
Level 3
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Securities purchased under agreements to resell		0
Fair Value, Nonrecurring
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Cash and cash equivalents	32,242	28,835
Deposits with clearing organizations	54,069	43,917
Securities borrowed	142,812	158,612
Receivables from brokers	58,041	65,639
Securities failed to deliver	32,006	29,656
Clearing organizations and other	43,680	30,780
Total receivable from brokers, dealers and clearing organizations	276,539	284,687
Receivable from customers	1,296,339	1,059,892
Notes receivable, net	65,580	62,640
Securities purchased under agreements to resell	0
Corporate-owned life insurance	98,068	88,989
Investments	1,469	2,010
Drafts payable	10,223	9,002
Bank call loans	206,700
Securities loaned	272,010	284,987
Payable to brokers	4,685	447
Securities failed to receive	35,200	23,809
Clearing organization and other	2,694	51,912
Total payables to brokers, dealers and clearing organizations	314,589	361,155
Payables to customers	367,905	369,287
Securities sold under agreements to repurchase	765,102	640,382
Senior secured notes	113,012	109,838
Fair Value, Nonrecurring | Level 1
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Cash and cash equivalents	32,242	28,835
Deposits with clearing organizations	54,069	43,917
Securities borrowed	0	0
Receivables from brokers	0	0
Securities failed to deliver	0	0
Clearing organizations and other	0	0
Total receivable from brokers, dealers and clearing organizations	0	0
Receivable from customers	0	0
Notes receivable, net	0	0
Securities purchased under agreements to resell	0
Corporate-owned life insurance	0	0
Investments	0	0
Drafts payable	10,223	9,002
Bank call loans	0
Securities loaned	0	0
Payable to brokers	0	0
Securities failed to receive	0	0
Clearing organization and other	0	0
Total payables to brokers, dealers and clearing organizations	0	0
Payables to customers	0	0
Securities sold under agreements to repurchase	0	0
Senior secured notes	0	0
Fair Value, Nonrecurring | Level 2
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Cash and cash equivalents	0	0
Deposits with clearing organizations	0	0
Securities borrowed	142,812	158,612
Receivables from brokers	58,041	65,639
Securities failed to deliver	32,006	29,656
Clearing organizations and other	43,680	30,780
Total receivable from brokers, dealers and clearing organizations	276,539	284,687
Receivable from customers	1,296,339	1,059,892
Notes receivable, net	65,580	62,640
Securities purchased under agreements to resell	0
Corporate-owned life insurance	98,068	88,989
Investments	1,469	2,010
Drafts payable	0	0
Bank call loans	206,700
Securities loaned	272,010	284,987
Payable to brokers	4,685	447
Securities failed to receive	35,200	23,809
Clearing organization and other	2,694	51,912
Total payables to brokers, dealers and clearing organizations	314,589	361,155
Payables to customers	367,905	369,287
Securities sold under agreements to repurchase	765,102	640,382
Senior secured notes	113,012	109,838
Fair Value, Nonrecurring | Level 3
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Cash and cash equivalents	0	0
Deposits with clearing organizations	0	0
Securities borrowed	0	0
Receivables from brokers	0	0
Securities failed to deliver	0	0
Clearing organizations and other	0	0
Total receivable from brokers, dealers and clearing organizations	0	0
Receivable from customers	0	0
Notes receivable, net	0	0
Securities purchased under agreements to resell	0
Corporate-owned life insurance	0	0
Investments	0	0
Drafts payable	0	0
Bank call loans	0
Securities loaned	0	0
Payable to brokers	0	0
Securities failed to receive	0	0
Clearing organization and other	0	0
Total payables to brokers, dealers and clearing organizations	0	0
Payables to customers	0	0
Securities sold under agreements to repurchase	0	0
Senior secured notes	0	0
Carrying Value | Fair Value, Nonrecurring
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Cash and cash equivalents	32,242	28,835
Deposits with clearing organizations	54,069	43,917
Securities borrowed	142,812	158,612
Receivables from brokers	58,041	65,639
Securities failed to deliver	32,006	29,656
Clearing organizations and other	43,680	30,780
Total receivable from brokers, dealers and clearing organizations	276,539	284,687
Receivable from customers	1,296,339	1,059,892
Notes receivable, net	65,580	62,640
Securities purchased under agreements to resell	0
Corporate-owned life insurance	98,068	88,989
Investments	1,469	2,010
Drafts payable	10,223	9,002
Bank call loans	206,700
Securities loaned	272,010	284,987
Payable to brokers	4,685	447
Securities failed to receive	35,200	23,809
Clearing organization and other	2,694	51,912
Total payables to brokers, dealers and clearing organizations	314,589	361,155
Payables to customers	367,905	369,287
Securities sold under agreements to repurchase	765,102	640,382
Senior secured notes	$ 113,050	$ 113,050